1. Technical Field
The invention relates to a windable multiwire parallel conductor used for windings of electrical devices and machines. The multiwire parallel conductor has several strands that are electrically insulated from each other and arranged as at least two juxtaposed stacks. The strands are bent in such a way that their position in the stacks is reversed over the length of the multiwire parallel conductor.
2. Description of the Related Art
Electrical conductors with a large cross-section for applications as windings of electrical devices and machines, a particular of transformers, are subdivided into a number of strands that are electrically insulated from each other and connected in parallel at their respective ends. These so-called twisted conductors can be quite long and are therefore capable of being wound on a cable reel. They have an approximately rectangular total cross section and are formed from a plurality of solid strands, with each strand having a flat, at least approximately rectangular cross-section and an electrical insulation. The strands are made of copper, aluminum or an alloy thereof. In order to keep the current displacement at a minimum, the position of the strands in the total cross-section of the multiwire parallel conductor can be repeatedly interchanged along the length of the twisted conductor. A suitable tool is used in a continuous manufacturing process to bend the strands at predetermined locations so that the position of the strands in the multiwire parallel conductor changes according to the bends and displacement of the strands. The changes in the position of the individual strands across the cross-section of the multiwire parallel conductor advantageously produces very small eddy current losses. The flat sides of the strands can be bent or offset, for example, around the longitudinal axis of the multiwire parallel conductor, while the order of the strands in the cross-section of the multiwire parallel conductor is maintained over several repeating cycles.
In transformers, but also in other electrical devices and machines, the electrical losses cause a considerable increase in temperature of the multiwire parallel conductors forming the windings. Consequently, the dissipated heat has to be removed from the coiled multiwire parallel conductors through an appropriate coolant. The efficiency of the cooling depends here very much on the thickness of the paper tape wrapping that typically surrounds the solid insulated strands of the multiwire parallel conductors. A thick paper wrapping results in a good electrical insulation of the multiwire parallel conductor. Optimal cooling of the multiwire parallel conductor, however, can only be achieved when any additional electrical insulation of the multiwire parallel conductor is eliminated entirely, i.e. when the multiwire parallel conductors are wound without any wrapping. However, this can be implemented for transformers only on the low voltage side.
Multiwire parallel conductors that have a paper insulation, are conventionally wound by inserting spacer plates between the windings. In this way, one or several oil passageways are formed to improve the cooling of the winding. However, the paper tapes may block the cross-section of the cooling channels, thereby limiting the circulation of the coolant. The low mechanical strength of the paper insulation must also be considered when the winding is manufactured.
EP 0 746 861 B1 describes a multiwire parallel conductor wherein the strands are surrounded by a woven ribbon. The weft threads and/or warp threads of the ribbon are made of polyester or a mixed yarn of glass fiber and polyester. To increase the mechanical strength of the covering, the woven ribbon can be in addition be pre-impregnated with a partially cross-linked epoxy resin.